Conventional wind turbines comprise a tower firmly anchored to the ground, a nacelle at the top of the tower, and a rotor equipped with at least one blade. Recent tendencies in wind turbine construction entail increased power and towers of greater height.
Wind turbine towers are constructed in sections, which comprise flanges and rings, that determine a thickness of 15-60 mm depending on the height of the tower and the rotor installed in the wind turbine. Each section is coupled to the adjacent section (upper and/or lower) by flanges that are bolted together.
The lower part of the wind turbine tower has an access door that enables workers to enter the wind turbine to undertake maintenance tasks. This access door is installed in a hollow or through opening made into the lower ring of the tower. The problem addressed here is the loss of ring strength and rigidness when this hollow is made.
Wind turbine tower manufacturers normally tend to reinforce this area with frames for the door or by increasing the thickness of the ring in the area, but this extra rigidness proves complex and expensive to produce.
An example of solutions of this sort is described in patent CA 2464623, which claims a structural member (frame) that is welded onto the hole made on the lower part of the tower to accommodate a door.
Other solutions are known such as the one described in patent EP 1856410, which proposes a lower ring manufactured in two parts onto which a door frame is mounted so that one of the parts of the ring, the one onto which the door will be mounted, will be thicker than the other part of the ring. This thus permits a thinner frame to be installed depending on the increase in ring thickness. However, this solution is inefficient, since it substantially increases the weight and mass of this lower tower ring.